Miller Goss

In two VLA proposals with Snezana Stanimirovic (Univ Wisconsin), Carl Heiles (UC-Berkeley), and Ayesha Begum (Univ Wisconsin), we have investigated weak galactic HI lines in the direction of the extragalactic sources AS931 and AB1325. These lines have opacities of several times 0.001. In the direction of 3C286 we confirmed the weak lines detected by Stanimirovic and Heiles at Arecibo with opacities of 0.005 to 0.006 (Fig 1).

For baselines using only VLA antennas, the bandpass stability (in 1.56 MHz total bandwidth mode) is typical of the previous VLA system, with fluctuations in the bandpass at the level of 0.002 in tau (left panel, Fig 1). But with the new EVLA optical fiber system and the VLA correlator, the bandpass is remarkably stable, i.e., to better than a few parts in 10,000 (right panel, Fig 1). New observations (May 2009) in the direction of additional sources confirm this stability; in particular, observations of 3C273 shows the tau=0.02 line easily with a bandpass that is flat at the level of a few times a part in 10,000.

Galactic HI absorption spectra towards 3C286. Left panel: Spectrum obtained using only VLA antennas. Right panel: Spectrum obtained using EVLA antennas. The VLA-only spectrum shows distortions at the level of 40 mJy/beam in the bandpass in spite of frequent bandpass calibration, compared with <16 mJy/beam for the EVLA antennas.